Mourinho also laid into the Premier League once again for United's packed schedule, saying their final game should be played on Saturday to give them more time to prepare for the Europa Leage final. It's just a lost battle.
The omission against Saints drew fears De Gea may never be seen in a United shirt again, but Mourinho has moved to quash those suggestions.
Sergio Romero produced a string of saves early in the second half, to keep Manchester United in the contest.
Romero, second choice to David De Gea in the Premier League but preferred in Europe's second-tier competition throughout the season, seems to have cemented his place for the showdown in Sweden's capital.
The result means United can finish no higher than sixth with one game remaining and must rely on winning the Europa League final against Ajax to seecure a Champions League berth.More news: HTC is building a stand-alone Daydream headset that runs on Vive tech
"He says it's not a tear from his experience. believing and trusting his feelings, he should be OK", said the manager. "I hope he goes soft on us".
"I think we played a good game against a good team, a team that tried to win because they try to finish well and will probably finish eighth and I think we did more than okay". Tells ( Wilfried) Zaha to go slow, leaves (Christian) Benteke at home.
Mourinho also said centre-back Eric Bailly, who is suspended for the final, would feature, as well as Paul Pogba, who has been granted compassionate leave following the death of his father last Friday.
Southampton manager Claude Puel refused to discuss his future at the club, which is the subject of speculation.
United make four changes in total from their defeat to Tottenham last time out, with Henrikh Mkhitaryan, Marouane Fellaini and Matteo Darmian returning to the starting XI.